Limit number of redirects per site to 15 #4433

#4433

#### Changes

* New access callback checks for number of existing redirects
* Description text appears on the CP > Settings "URL Redirects" section
* The "+ Add new redirect" link doesn't appear after the user has hit the limit.

##### Screenshot: new description text

![screen shot 2013-12-12 at 3 39 36 pm](https://f.cloud.github.com/assets/228733/1737123/9f2dd138-636d-11e3-90de-f22cbcc533f7.png)